A mega-agency is a large advertising firm that is formed by merging several individual agencies. These types of agencies provide advertising and public relations services. Mega-agencies usually maintain regional offices worldwide, and many operate in-house radio and TV production studios.

There are several concerns about mega-agencies. One concern is that the large agencies are a threat to smaller independent firms. Another concern is that a few firms now control the distribution of ad dollars globally, and the cultural values represented by American and European ads may undermine the values and products of developing countries.